The molecular structures of the polyamideimide resin vary depending on the monomers employed. Typical example thereof is that represented by the following formula (I) ##STR1## wherein R is ##STR2## prepared by polycondensation of trimellitic acid anhydride (or its derivatives) as an aromatic tricarboxylic acid component with m-phenylene diamine and diaminodiphenyl ether as an aromatic diamine component.
Polyamideimide resin of the formula (I) is disclosed in U.S. Pat. No. 4,016,140 and Japanese Patent Laid-Open No. Hei 02-18,422. It is a transparent, amorphous resin which has the following properties:
(1) It has high heat distortion temperature of 278.degree. C. and a long term service temperature exceeding 200.degree. C. It has excellent heat resistance. So it can be used in wide range of temperature up to 260.degree. C.
(2) Even at high temperature exceeding 200.degree. C., it has excellent physical and mechanical properties, which are comparable to those of general purpose engineering plastics at room temperature. Moreover, it has good impact resistance.
(3) It has outstanding creep resistance.
(4) It has very low linear expansion coefficient of 4.times.10.sup.-5 cm/cm. .degree.C., which can be further reduced to less than half by using fillers.
(5) It has excellent electrical breakdown strength and volume resistivity, and shows flame retardance of UL 94 V-O without using flame retardants.
(6) If polyamideimide is compounded with PTFE or graphite, it shows good abrasion resistance and self lubricating properties. Therefore, it is suitable as sliding member under severe circumstance.
(7) It has good chemical resistance. That is, it is fairly stable in hydrocarbons. But care must be taken for concentrated aqueous alkali solution.
(8) It has good ultraviolet light resistance and radiation resistance.
Examples of the methods for preparing polyamideimide resins generally include the isocyanate method and acid chloride method.
The isocyanate method comprises the condensation of aromatic diisocyanate with aromatic tricarboxylic acid anhydride to give polyamideimide without via polyamic acid which is intermediate polymer as disclosed in Japanese Patent Publication No. Sho 44-19274 and U.S. Pat. No. 3,541,038 (1970).
The acid chloride method comprises the condensation of aromatic tricarboxylic acid chloride with aromatic diamine. This method is classified into "low temperature homogeneous solution polymerization method" and "low temperature precipitating polymerization method". The typical example of the low temperature homogeneous solution polymerization method comprises the polymerization reaction at room temperature in nonaqueous polar solvent such as N,N'-dimethylacetamide which was developed by Standard Oil Co., in the U.S.A as disclosed in U.S. Pat. No. 3,920,612 (1975). The typical example of the low temperature precipitating polymerization method comprises the polymerization reaction in an organic solvent which is sparingly soluble in water, such as methyl ethyl ketone (for example, produced by Teijin Kasei Corp. in Japan) and in an aqueous solvent by using triethylamine as an acid acceptor as disclosed in Japanese Patent Publication No. Sho 46-15,513. This reaction is a kind of interfacial polymerization method.
Another method of preparing polyamideimide resin is the direct polymerization method which comprises direct polymerization of aromatic diamine with aromatic tricarboxylic acid in the presence of dehydration catalyst as disclosed in U.S. Pat. No. 3,860,559 (1975) and Japanese Patent Laid-Open No. Sho 58-180532.
However, the isocyanate method has problems in that gelation occurs during the reaction and it is difficult to get linear high molecular weight polymers due to the formation of by-products. Therefore, the polyamideimide resin prepared by this method have poor melt flowability, melt processability, mechanical properties, and heat resistance, and thus is not suitable for application as injection molded articles.
Although it is possible to obtain sufficiently high molecular weight polyamideimide by the low temperature homogeneous solution polymerization method, you should have to use acid chloride as the raw material which is 5 to 10 times expensive than corresponding acid. Therefore, the price of the resulting polyamideimide is very high. Furthermore, since this method is carried out in two steps consisting of preparing polyamic acid as a primary polymer and then imidization of the latter by heating or by using dehydration agent. Moreover, this method also has a problem in that the modification of the molecular structure of the resins is almost impossible. Thus this method has little economic merit.
In the low temperature precipitating polymerization method, the expensive acid chloride is also used as the raw material. And the polyamic acid is precipitated out using water/methyl ethyl ketone mixed non solvent followed by cyclization. The polyamideimide resin prepared by this method has lower molecular weight with a large molecular weight distribution. Thus, the method is also impractical.
Both the isocyanate method and acid chloride method have disadvantages in that the handling of acid chloride and diisocyanate is troublesome since acid chloride and diisocyanate are sensitive to water, which should be blocked in the reaction process.
Meanwhile, the direct polymerization method of polyamideimide resins comprises directly polymerizing the aromatic diamine with aromatic tricarboxylic acid anhydride (or its derivatives) in the presence of polymerization dehydrating catalyst. The advantages of this polymerization method are that the process of this method is simplest among many preparing processes, the cost for raw materials and processing is not high, and that the handling of monomers is easy. For these reasons active researches have been made in this field. The examples of polymerization catalyst used in the method are phosphoric acid types such as phosphoric acid and polyphosphoric acid (Japanese Patent Publication Nos. Sho 63-27,527, Japanese Patent laid-open Nos. Sho 62-297,329 and Sho 63-108,027 and Japanese Patent Laid-open No. Hei 02-115,229), boric acid types such as boric acid and boric acid anhydride (French Patent No. 1,515,066 and Japanese Patent Laid-Open No Sho 58-180,532) or triphenyl phosphite and phosphoric triester type (U.S. Pat. No. 3,860,559) or two or more combination of those mentioned catalyst (Japanese Patent Laid-Open No. Sho 64-51438). The effect of these polymerization catalysts depends on the types of catalysts. However, to obtain polymer having high molecular weight by using costly polymerization catalyst and reactant monomers in the same molar ratio, it is necessary to carry out the reaction at high temperatures of 200.degree. C. or more for a long time. Thus, even if in the case of using a high boiling point solvent such as N-methyl pyrrolidone, sulfolan and nitrobenzene as a synthesizing solvent, tar-state substance formed by decomposition of monomers and generated resins in reaction vessel, and polymerization catalyst used in a large amount are incorporated into the polymer, which cause unsatisfactory color and deterioration of physical properties of polyamideimide. Moreover, this process is disadvantageous in that it is difficult to give linear polymers having high molecular weights due to side reaction and thereby the solubility of polymers is decreased. Particularly, this process has no economic merit since high costly polymerization catalyst has to be used in a large quantity and the recovery of this catalyst is impossible.
S. Maiti and A. Ray suggested a method for preparing polyamideimide resins which comprises polymerization at 0-5.degree. C. by using N,N-dimethylformamide containing thionyl chloride and lithium chloride as solvents and pyridine as an acid acceptor as disclosed in Makromol. Chem. Rapid Commun. 2, 649-653 (1981). However, this method has no economic merit since costly metal salts are used in the method and polyamideimides having low molecular weights are obtained by the method.
Based on the problems of the above-mentioned processes, the present inventors have made extensive studies in order to solve the problems of the known direct polymerization method and to find a process for preparing polyamideimide resins having high molecular weights with good heat resistance, melt flowability, solubility and economic merit. As the result, the present inventors have now found that by dissolving an aromatic tricarboxylic acid anhydride and an aromatic diamine in a polar solvent, subjecting the resulting diimide dicarboxylic acid to acyl halogenating agent treatment to give an intermediate having good reactivity and then adding diamine as a nucleophilic agent, polyamideimide resins having high molecular weights can be prepared. The present invention has been attained on the basis of this finding.